CNN — Russia showered Kyiv, Lviv and other major cities across Ukraine with what officials said was an unprecedented array of missiles on Thursday morning, stepping up its assault on the entire country as a sluggish ground war drags on in the east.

A total of 81 missiles were used in a “massive attack” on Ukrainian infrastructure, including six Kinzhal ballistic missiles that have the ability to elude Kyiv’s air defenses, the Ukrainian military said.

“The attack is really large-scale and for the first time using such different types of missiles. We see that this time as many as six Kinzhal were used. This is an attack like I don’t remember seeing before,” Yurii Ihnat, spokesman for the Air Force Command of Ukraine, said on Ukrainian television Thursday.

“So far, we have no capabilities to counter these weapons,” he added, referring to the Kinzhals, plus six X-22 air-launched cruise missiles that were also launched by Russian forces.

“It’s been a difficult night,” President Volodymyr Zelensky said Thursday in a Facebook message.

“The enemy fired 81 missiles in an attempt to intimidate Ukrainians again, returning to their miserable tactics. The occupiers can only terrorize civilians. That’s all they can do. But it won’t help them. They won’t avoid responsibility for everything they have done,” Zelensky said.

He listed 10 regions across Ukraine where aerial attacks took place, including Dnipro, Odesa, Kharkiv and Zaporizhzhia, and said the attacks hit “critical infrastructure and residential buildings.”

“Unfortunately, there are injured and dead. My condolences to the families,” he added.

Russia used the nuclear-capable Kinzhal missile, which it has described as a hypersonic weapon, on a few occasions in the first weeks of its invasion last year. But the powerful weapon, which Ukraine doesn’t have the capability to shoot down, has rarely been seen over the country’s skies.

At least 16 people were killed and more than 20 injured during the overnight attacks, according to preliminary information from regional authorities.

In Kyiv, an air raid alert lasted for almost 7 hours overnight into Thursday and power outages were implemented as a preventative measure, regional authorities said. In the Zolochiv community near Lviv, a fire broke out when the fragments of a Russian missile were shot down, regional authorities said.

The fire destroyed three residential buildings, and three cars. The rubble was being cleared and rescuers were searching for additional victims on Thursday morning. Several infrastructure facilities and other buildings were hit elsewhere in Ukraine.

The Russian Ministry of Defense said Thursday the barrage of missile strikes it launched was retaliation for what the ministry called “terrorist actions” organized by Kyiv in Russia’s Bryansk region last week.

“In response to the terrorist actions in the Bryansk region organized by the Kyiv regime on March 2 this year, the Armed Forces of the Russian Federation delivered a massive retaliation strike,” it said in a statement.

“High-precision long-range air, sea and land-based weapons, including the Kinzhal hypersonic missile system, hit key elements of Ukraine’s military infrastructure, military-industrial complex enterprises, as well as energy facilities that serve them,” the ministry said.

Russian security officials claimed a small Ukrainian armed group last week crossed the Russian border into the southern Bryansk region. Russia’s Federal Security Service (FSB) said the agency was carrying out operations following “armed Ukrainian nationalists who violated the state border.” Russian President Vladimir Putin described the incident as a “terrorist attack.” A local official said two civilians were killed.

CNN cannot independently verify the Russian claims, and local media did not carry any images of the supposed incidents, any type of confrontation or an alleged raid reported by Russian authorities.

Rare use of Kinzhal missile

The use of such a wide and unpredictable array of weaponry seemingly marks a shift in the Kremlin’s strategy.

The Kinzhal, an air-launched variant of the Iskander short-range ballistic missile (SRBM) which has also, more frequently, been used in Ukraine, was unveiled by Putin in 2018 as a cornerstone of a modernized Russian arsenal.

Like virtually all ballistic missiles it is hypersonic, which means they travel at least five times the speed of sound, but it is also particularly difficult to detect because it can be launched from MiG-31 fighter jets, giving it a longer range and the ability to attack from multiple directions.

“Russia likely developed the unique missile to more easily target critical European infrastructure … (its) speed, in combination with the missile’s erratic flight trajectory and high maneuverability, could complicate interception,” according to the Center for Strategic and International Studies (CSIS).

Russia’s use of the missile on Ukrainian targets last March was its first known use in combat, according to CSIS, and it was subsequently used again in May.

Eight Iranian-made Shahed drones were also used in Thursday’s attacks, authorities said. A senior US defense official said Thursday that Ukraine is becoming a “battle lab” for testing Iranian weapons outside of the Middle East. The official spoke ahead of US Secretary of Defense Lloyd Austin’s visit to Israel where Iranian-Russian military cooperation will be on the agenda.

“Everyone should be preparing for what the threat scenarios look like when Iran takes the tactics, techniques and procedures it learned in Ukraine and starts to use those coercive tactics here,” the official said in reference to the Middle East.

The barrage came as most focus in Ukraine was fixed to Bakhmut, the eastern city that Russia’s ground forces have been assaulting for weeks and appear to be on the cusp of capturing.

Ukraine’s troops have sustained a determined defense of the city even as some military experts advocate for a tactical withdrawal.

Zelensky said in an interview with CNN on Tuesday that Kyiv’s ongoing resistance in the city is “tactical,” warning that Russians could advance towards other key cities to the west if they capture Bakhmut.

“We understand that after Bakhmut they could go further. They could go to Kramatorsk, they could go to Sloviansk, it would be open road for the Russians after Bakhmut to other towns in Ukraine, in the Donetsk direction,” he told CNN’s Wolf Blitzer in an exclusive interview from Kyiv. “That’s why our guys are standing there.”

Correction: This story has been updated to clarify that virtually all ballistic missiles — not missiles in general — are hypersonic.

CNN’s Svitlana Vlasova and Radina Gigova contributed reporting.